1. Field of the Invention
The present invention relates to an axial-force-detective fastening tool for fastening bolts and nuts to, for example, a suspension of an automobile. The present invention also relates to a bolt to be fastened with the axial-force-detective fastening tool and a method of manufacturing the bolt.
2. Description of Related Art
FIG. 12 shows an example of a conventional axial-force-detective fastening tool.
The fastening tool of FIG. 12 has a detective socket 201 removably attached to a torque wrench 203. The detective socket 201 has a socket body 205 and a stationary member 207. The socket body 205 is rotatable relative to the stationary member 207. The socket body 205 has a bolt receiving hole 209 that receives a head 212 of a bolt 211. On the back side of the bolt receiving hole 209, an ultrasonic sensor 213 is arranged.
The ultrasonic sensor 213 has a permanent magnet body 214 and a piezoelectric element 215. The piezoelectric element 215 directly comes in contact with the bolt head 212. The piezoelectric element 215 is connected to a lead wire 217, which is connected to a terminal of a slip ring 219. Another terminal of the slip ring 219 is connected to a coaxial cable 221 supported with the stationary member 207.
The torque wrench 203 has a drive shaft 223 and a ratchet mechanism 225 driven through meshing bevel gears 227 and 229. The ratchet mechanism 225 is interlinked with a shaft 231 engaging with a top end of the socket body 205 through a pin 233.
When the bolt receiving hole 209 of the socket body 205 is set on the head 212 of the bolt 211, the bolt head 212 is attracted by the magnet body 214 so that the piezoelectric element 215 is brought into contact with the top face of the bolt head 212.
Thereafter, the drive shaft 223 is driven to rotate the shaft 231 through the bevel gears 227 and 229 and ratchet mechanism 225. Together with the shaft 231, the socket body 205 rotates relative to the stationary member 207. The rotation of the socket body 205 turns the head 212 of the bolt 211 so that the bolt 211 is screwed into a block 235 to fasten an object 237.
The fastening force of the bolt 211 is controlled by managing the axial force of the bolt 211. Namely, a controller (not shown) makes the piezoelectric element 215 emit ultrasonic waves, which are reflected by a front end of the bolt 211 and are received by the piezoelectric element 215. The controller measures an elapsed time between the emission of the ultrasonic waves and the reception thereof as a round-trip time of the ultrasonic waves along an axial length of the bolt 211. Based on the round-trip time, the controller computes the axial length of the bolt 211, and then, an axial length extension ratio of the bolt 211 due to the fastening. Thereafter, the controller computes an axial force that is proportional to the extension ratio. When the axial force reaches a set value, the controller terminates the fastening of the bolt 211. As a result, the bolt 211 is correctly fastened according to an axial force acting on the bolt 211.
The related art mentioned above may be effective when directly fastening the bolt 211 to the block 235. The related art, however, is unsatisfactory when fastening a nut to the bolt 211 because the bolt 211 turns with the nut when the socket body 205 fastens the nut to the bolt 211. Namely, it is incapable for the fastening tool of the related art to fasten a nut while measuring an axial force with the fastening tool being held with one hand of an operator.
If the bolt 211 is a screw stud, a nut can be fastened to the bolt without corotation. In this case, however, a front end of the bolt turns relative to the piezoelectric element 215 when the nut is fastened, to break the piezoelectric element 215. Namely, it is practically difficult for the related art to fasten a nut while measuring an axial force acting on a bolt.
When the related art fastens a nut to a bolt, a front end of the bolt protrudes from the nut as the nut is fastened to the bolt. The ultrasonic sensor 213 fixed to the socket body 205 is unable to cope with this situation. Namely, the ultrasonic sensor 213 never allows the front end of the bolt to protrude toward the ultrasonic sensor 213.
The piezoelectric element 215 is fixed to the permanent magnet body 214. Accordingly, a face of the magnet body 214 that comes in contact with the head 212 of the bolt 211 must be aligned with a face of the piezoelectric element 215 that also comes in contact with the bolt head 212. If there is a misalignment between them, the piezoelectric element 215 may incorrectly contact the bolt head 212 when the magnetic body 214 correctly contact the bolt head 212. Namely, the magnetic body 214 and piezoelectric element 215 of the related art require very complicated and difficult assembling work.
The ultrasonic sensor 213 is fixed to the socket body 205, and therefore, the piezoelectric element 215 and controller must transfer signals between them through the socket body 205 and stationary member 207. Due to this, the lead wire 217 and coaxial cable 221 must be connected to each other through the slip ring 219. This is a complicated structure. In addition, the durability of the slip ring 219 is low.
The stationary member 207 is interposed between the socket body 205 and the torque wrench 203. When the related art is used in a car assembling process to fasten a nut to parts located at an inside position, such as a suspension slat and a knuckle arm, the socket body 205 is hardly inserted into the inside position because the stationary member 207 prevents the same. In this way, the related art has operational limits and difficulties.